Method and apparatus for performing handover in wireless communication system

ABSTRACT

A method for supporting handover of a moving entity by a serving base station comprises transmitting a measurement configuration message including measurement configuration regarding neighboring base stations, to the moving entity; receiving a handover accept message from a target base station, to which the moving entity will perform handover, among the neighboring base stations; and transmitting a radio resource control (RRC) reconfiguration message indicating reconfiguration of RRC connection, to the moving entity, wherein a handover initiation scheme and a target base station selection scheme are determined according to whether the moving entity is a mobile terminal or a moving cell.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT International ApplicationNo. PCT/KR2015/001619, filed on Feb. 17, 2015, which claims priorityunder 35 U.S.C. 119(e) to U.S. Provisional Application Nos. 61/948,552,filed on Mar. 6, 2014 and 61/952,883, filed on Mar. 14, 2014, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for performing a handoverby a moving cell.

BACKGROUND ART

A wireless communication system has been widely developed to providevarious kinds of communication services such as voice and data.Generally, the wireless communication system is a multiple access systemthat can support communication with multiple users by sharing availablesystem resources (bandwidth, transmission power, etc.). Examples of themultiple access system include a code division multiple access (CDMA)system, a frequency division multiple access (FDMA) system, a timedivision multiple access (TDMA) system, an orthogonal frequency divisionmultiple access (OFDMA) system, a single carrier frequency divisionmultiple access (SC-FDMA) system, and a multi carrier frequency divisionmultiple access (MC-FDMA).

Recently, a radio access network structure has been changed to astructure that various types of small cells, for example, pico cells orfemto cells interact with a macro cell. As a cell structure ismulti-layered, a data transmission rate and QoE may be improved. In the3GPP, indoor/outdoor scenarios based on low power nodes to improve smallcells have been discussed. This discussion is disclosed in the 3GPP TR36.932. Also, dual connectivity for a macro cell and the small cells hasbeen discussed. As described above, in the future wireless communicationenvironment, as many small cells are used, it is expected that a userequipment and cells are located at a physically closer distance.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method for a movingcell to perform handover rapidly by efficiently acquiring information ofa target cell, which is required for the moving cell to perform handoverto the target cell.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

According to one aspect of the present invention, a method forsupporting handover of a moving entity by a serving base stationcomprises transmitting a measurement configuration message includingmeasurement configuration regarding neighboring base stations, to themoving entity; receiving a handover accept message from a target basestation, to which the moving entity will perform handover, among theneighboring base stations; and transmitting a radio resource control(RRC) reconfiguration message indicating reconfiguration of RRCconnection, to the moving entity, wherein a handover initiation schemeand a target base station selection scheme are determined according towhether the moving entity is a mobile terminal or a moving cell.

According to another aspect of the present invention, a method forperforming a handover by a moving cell comprises measuring neighboringbase stations based on a measurement configuration message received froma serving base station; determining initiation of handover and a targetbase station in accordance with a result of the measurement of theneighboring base stations; and transmitting a random access preamble tothe target base station in a state that radio resource control (RRC)connection to the serving base station is maintained, wherein, if amobile terminal served by the moving cell performs handover, the movingcell releases RRC connection of the mobile terminal before the mobileterminal transmits the random access preamble.

According to other aspect of the present invention, a serving basestation for supporting handover of a moving entity comprises a radiointerface transmitting a measurement configuration message includingmeasurement configuration regarding neighboring base stations, to themoving entity and transmitting a radio resource control (RRC)reconfiguration message indicating reconfiguration of RRC connection, tothe moving entity; a backhaul interface receiving a handover acceptmessage from a target base station, to which the moving entity willperform handover, among the neighboring base stations; and a processorcontrolling the radio interface and the backhaul interface, wherein ahandover initiation scheme and a target base station selection schemeare determined according to whether the moving entity is a mobileterminal or a moving cell.

Advantageous Effects

According to one embodiment of the present invention, a moving cell maydetermine handover by itself and efficiently acquire information of atarget cell, whereby the time required for handover may be reduced, anddata interruption of the moving cell, which is caused by handover, maybe minimized.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a structure of a radio frame;

FIG. 2 is a diagram illustrating a resource grid at a downlink slot;

FIG. 3 is a diagram illustrating a structure of a downlink subframe;

FIG. 4 is a diagram illustrating a structure of an uplink subframe;

FIG. 5 is a diagram illustrating a primary synchronous signal(PSS)/secondary synchronous signal (SSS) in an FDD system;

FIG. 6 is a diagram illustrating a physical broadcast channel (PBCH);

FIG. 7 is a diagram illustrating a handover procedure of a userequipment;

FIG. 8 is a diagram illustrating a handover procedure of a moving cellaccording to one embodiment of the present invention;

FIG. 9 is a diagram illustrating a handover procedure of a moving cellaccording to another embodiment of the present invention;

FIG. 10 is a diagram illustrating a moving track of a moving cellaccording to one embodiment of the present invention;

FIG. 11 is a diagram illustrating a handover pre-processing procedure ofa moving cell according to one embodiment of the present invention;

FIG. 12 is a diagram illustrating a moving track of a moving cellaccording to another embodiment of the present invention;

FIG. 13 is a diagram illustrating a handover pre-processing procedure ofa moving cell according to another embodiment of the present invention;

FIG. 14 is a diagram illustrating a handover procedure of a moving cellaccording to other embodiment of the present invention;

FIG. 15 is a diagram illustrating a handover pre-processing procedure ofa moving cell according to other embodiment of the present invention;

FIG. 16 is a diagram illustrating a cell according to one embodiment ofthe present invention; and

FIG. 17 is a diagram illustrating a structure of a user equipment and abase station according to one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment.

In the embodiments of the present invention, a description is made,centering on a data transmission and reception relationship between abase station (BS) and a user equipment (UE). The BS is a terminal nodeof a network, which communicates directly with a UE. In some cases, aspecific operation described as performed by the BS may be performed byan upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality ofnetwork nodes including a BS, various operations performed forcommunication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with the term ‘fixedstation’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘Access Point(AP)’, etc. The term ‘UE’ may be replaced with the term ‘terminal’,‘Mobile Station (MS)’, ‘Mobile Subscriber Station (MSS)’, ‘SubscriberStation (SS)’, etc.

Specific terms used for the embodiments of the present invention areprovided to help the understanding of the present invention. Thesespecific terms may be replaced with other terms within the scope andspirit of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention can be supported by standarddocuments disclosed for at least one of wireless access systems,Institute of Electrical and Electronics Engineers (IEEE) 802, 3rdGeneration Partnership Project (3GPP), 3GPP Long Term Evolution (3GPPLTE), LTE-Advanced (LTE-A), and 3GPP2. Steps or parts that are notdescribed to clarify the technical features of the present invention canbe supported by those documents. Further, all terms as set forth hereincan be explained by the standard documents.

Techniques described herein can be used in various wireless accesssystems such as Code Division Multiple Access (CDMA), Frequency DivisionMultiple Access (FDMA), Time Division Multiple Access (TDMA), OrthogonalFrequency Division Multiple Access (OFDMA), Single Carrier-FrequencyDivision Multiple Access (SC-FDMA), etc. CDMA may be implemented as aradio technology such as Universal Terrestrial Radio Access (UTRA) orCDMA2000. TDMA may be implemented as a radio technology such as GlobalSystem for Mobile communications (GSM)/General Packet Radio Service(GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may beimplemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802.20, Evolved-UTRA (E-UTRA) etc. UTRA is a partof Universal Mobile Telecommunication System (UMTS). 3GPP LTE is a partof Evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA fordownlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE.WiMAX can be described by the IEEE 802.16e standard (WirelessMetropolitan Area Network (WirelessMAN-OFDMA Reference System) and theIEEE 802.16m standard (WirelessMAN-OFDMA Advanced System). For clarity,this application focuses on the 3GPP LTE/LTE-A system. However, thetechnical features of the present invention are not limited thereto.

A description will be given of a radio frame structure of 3GPP LTE withreference to FIG. 1.

In a cellular OFDM wireless packet communication system, uplink/downlinkdata packet transmission is performed on a subframe-by-subframe basisand one subframe is defined as a predetermined time interval including aplurality of OFDM symbols. 3GPP LTE supports type-1 radio frameapplicable to FDD (frequency division duplex) and type-2 radio frameapplicable to TDD (time division duplex).

FIG. 1(a) illustrates a type-1 radio frame structure. A downlink radioframe includes 10 subframes. Each subframe is further divided into twoslots in the time domain. A unit time during which one subframe istransmitted is defined as transmission time interval (TTI). For example,one subframe may be 1 ms in duration and one slot may be 0.5 ms induration. A slot may include a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Since 3GPP LTE adopts OFDMA for downlink, an OFDM symbol represents onesymbol period. An OFDM symbol may be referred to as an SC-FDMA symbol orsymbol period. A resource block (RB) is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols included in one slot may depend on cyclicprefix (CP) configuration. CPs include an extended CP and a normal CP.When an OFDM symbol is configured with the normal CP, for example, thenumber of OFDM symbols included in one slot may be 7. When an OFDMsymbol is configured with the extended CP, the duration of one OFDMsymbol increases, and thus the number of OFDM symbols included in oneslot is smaller than that in case of the normal CP. In case of theextended CP, the number of OFDM symbols allocated to one slot may be 6.When a channel state is unstable, such as a case in which a UE moves ata high speed, the extended CP can be used to reduce inter-symbolinterference.

FIG. 1(b) illustrates a type-2 radio frame structure. The type-2 radioframe includes 2 half frames. Each half frame includes 5 subframes, adownlink pilot time slot (DwPTS), a guard period (GP) and an uplinkpilot time slot (UpPTS). One subframe consists of 2 slots. The DwPTS isused for initial cell search, synchronization or channel estimation in aUE. The UpPTS is used for channel estimation in a BS and UL transmissionsynchronization acquisition in a UE. The GP eliminates UL interferencecaused by multi-path delay of a DL signal between a UL and a DL. Onesubframe includes 2 slots irrespective of radio frame type.

The radio frame structure is purely exemplary and thus the number ofsubframes in a radio frame; the number of slots in a subframe, or thenumber of OFDM symbols in a slot may vary.

FIG. 2 illustrates a resource grid in a downlink slot. While onedownlink slot includes 7 OFDM symbols in the time domain and one RBincludes 12 subcarriers in the frequency domain in FIG. 2, the presentinvention is not limited thereto. For example, one slot includes 7 OFDMsymbols in the case of normal CP whereas one slot includes 6 OFDMsymbols in the case of extended CP. Each element on the resource grid isreferred to as a resource element (RE). One RB includes 12×7 REs. Thenumber NDL of RBs included in the downlink slot depends on a downlinktransmit bandwidth. The structure of an uplink slot may be same as thatof the downlink slot.

FIG. 3 illustrates a downlink subframe structure. A maximum of threeOFDM symbols located in a front portion of a first slot within asubframe correspond to a control region to which a control channel isallocated. The remaining OFDM symbols correspond to a data region towhich a physical downlink shared chancel (PDSCH) is allocated. Examplesof downlink control channels used in 3GPP LTE include a physical controlformat indicator channel (PCFICH), a physical downlink control channel(PDCCH), a physical hybrid ARQ indicator channel (PHICH), etc. ThePCFICH is transmitted at a first OFDM symbol of a subframe and carriesinformation regarding the number of OFDM symbols used for transmissionof control channels within the subframe. The PHICH is a response ofuplink transmission and carries an HARQ acknowledgment(ACK)/negative-acknowledgment (NACK) signal. Control informationtransmitted through the PDCCH is referred to as downlink controlinformation (DCI). The DCI includes uplink or downlink schedulinginformation or uplink Tx power control commands for an arbitrary UEgroup. The PDCCH may carry a transport format and a resource allocationof a downlink shared channel (DL-SCH), resource allocation informationof an uplink shared channel (UL-SCH), paging information on a pagingchannel (PCH), system information on the DL-SCH, information on resourceallocation of an upper-layer control message such as a random accessresponse transmitted on the PDSCH, a set of Tx power control commands onindividual UEs within an arbitrary UE group, a Tx power control command,information on activation of a voice over IP (VoIP), etc. A plurality ofPDCCHs can be transmitted within a control region. The UE can monitorthe plurality of PDCCHs. The PDCCH is transmitted on an aggregation ofone or several consecutive control channel elements (CCEs). The CCE is alogical allocation unit used to provide the PDCCH with a coding ratebased on a state of a radio channel. The CCE corresponds to a pluralityof resource element groups (REGs). A format of the PDCCH and the numberof bits of the available PDCCH are determined by the number of CCEs. TheBS determines a PDCCH format according to DCI to be transmitted to theUE, and attaches a cyclic redundancy check (CRC) to control information.The CRC is masked with an identifier referred to as a radio networktemporary identifier (RNTI) according to an owner or usage of the PDCCH.If the PDCCH is for a specific UE, a cell-RNTI (C-RNTI)) of the UE maybe masked to the CRC. Alternatively, when the PDCCH is for a pagingmessage, a paging indicator identifier (P-RNTI) may be masked to theCRC. When the PDCCH is for system information (more specifically, asystem information block (SIB)), a system information identifier andsystem information RNTI (SI-RNTI) may be masked to the CRC. To indicatea random access response corresponding to a response to transmission ofa random access preamble of the UE, a random access-RNTI (RA-RNTI) maybe masked to the CRC.

FIG. 4 illustrates an uplink subframe structure. An uplink subframe maybe divided into a control region and a data region in the frequencydomain. The control region is allocated a PUCCH including uplink controlinformation. The data region is allocated a PUSCH including user data.To maintain single carrier property, one UE cannot simultaneouslytransmit a PUCCH and a PUSCH. A PUCCH for a UE is allocated to an RBpair. RBs belonging to an RB pair occupy different subcarriers in 2slots. That is, an RB pair allocated to a PUCCH is frequency-hopped at aslot boundary.

PSS (Primary Synchronous Signal)/SSS (Secondary Synchronous Signal)

FIG. 5 is a diagram illustrating a PSS and an SSS, which are synchronoussignals used for cell search in an LTE/LTE-A system. Cell search will bedescribed prior to description of the PSS and the SSS. When the userequipment is initially connected to a cell, cell search is performed ifa user equipment performs handover from a cell, to which the userequipment is currently connected, to another cell, or is performed forcell reselection. Cell search may be performed by frequency and symbolsynchronous acquisition of the cell, downlink frame synchronousacquisition of the cell, and determination of cell identifier (ID).Three cell IDs may constitute one cell group, and 168 cell groups mayexist.

For cell search, a base station transmits the PSS and the SSS. The userequipment may acquire 5 ms timing of the cell by detecting the PSS andidentify cell ID within the cell group. Also, the user equipment mayidentify radio frame timing and cell group by detecting the SSS.

Referring to FIG. 5, the PSS is transmitted from the subframes 0 and 5.In more detail, the PSS is transmitted from the last OFDM symbol of thefirst slot at the subframes 0 and 5. Also, the SSS is transmitted fromthe last second OFDM symbol of the first slot of the subframes 0 and 5.That is, the SSS is transmitted from the OFDM symbol just before the PSSis transmitted. This transmission timing corresponds to FDD (FrequencyDivision Duplex). In case of TDD (Time Division Duplex), the PSS istransmitted from the third symbol of the subframes 1 and 6, that is,DwPTS, and the SSS is transmitted from the last symbol of the subframes0 and 5. That is, in the TDD, the SSS is transmitted prior to the PSS asmuch as three symbols.

The PSS is a Zadoff-Chu sequence of length 63, and in real transmission,0 is padded at both ends of the sequence, whereby the sequence istransmitted onto 73 subcarriers (72 subcarriers except DC subcarrier,that is, 6 RBs) in the middle of a system frequency bandwidth. The SSSis a sequence of length 62, which is obtained as two sequences of length31 are frequency-interleaved, and is transmitted onto 72 subcarriers inthe middle of a full system bandwidth in the same manner as the PSS.

PBCH (Physical Broadcast Channel)

FIG. 6 is a diagram illustrating a physical broadcast channel (PBCH).The PBCH is the channel to which system information corresponding to amaster information block (MIB) is transmitted, and is used to allow theuser equipment to system information after acquiring downlinksynchronization and cell ID through the aforementioned PSS/SSS. In thiscase, the MIB may include downlink cell bandwidth information, PHICHconfiguration information, subframe number (SFN), etc.

One MIB, as shown in FIG. 6, is transmitted through a first subframe ofeach of four continuous radio frames. In more detail, the PBCH istransmitted from first four OFDM symbols of the second slot of thesubframe 0 at four continuous radio frames. Accordingly, the PBCH fortransmitting one MIB is transmitted at a period of 40 ms. The PBCH istransmitted on 72 subcarriers in the middle of the full bandwidth on afrequency axis, which correspond to the smallest downlink bandwidth, 6RBs. This is to allow the user equipment to decode BCH without problemeven though the user equipment does not know the size of the full systembandwidth.

Handover of User Equipment (UE)

FIG. 7 is a diagram illustrating a handover procedure of a userequipment.

In FIG. 7, a network system may include a user equipment, a source basestation (eNB), and a target base station (eNB). At this time, the sourcebase station is a serving base station (or serving cell) that provides ascheduling service to the user equipment, and the target base station(or target cell) is the base station that will provide a schedulingservice to the user equipment after handover is completed. Also, thesource base station and the target base station may be fixed cells ormoving cells. For example, the user equipment may perform handover fromthe fixed cell to the moving cell or move from the moving cell to thefixed cell. The user equipment may perform handover to a homogeneouscell. The fixed cell may be understood as a legacy cell, and the movingcell is a cell which is newly defined by the embodiments of the presentinvention and will be described in detail later.

Referring to FIG. 7, the user equipment transmits a measurement reportmessage, which includes a measurement result of a neighboring cell, tothe source base station (S601). In this case, the measurement reportmessage may include reference signal received power (RSRP), receivedsignal strength indicator (RSSI), and reference signal received quality(RSRQ). The RSRP is a measurement value that may be obtained bymeasuring a size of a downlink RS. The RSSI is a total received powervalue of the user equipment, and is a measurement value that includesinterference from neighboring cells and noise power. The RSRQ is a valueacquired based on N*RSRP/RSSI, wherein N is the number of RBs of abandwidth during RSSI measurement.

Transmission of measurement report may be determined by event-basedmeasurement report determination as follows. The event for measurementreport determination may include, but not limited to, at least one of i)the event that a measurement value of the serving cell becomes greaterthan an absolute threshold value, ii) the event that the measurementvalue of the serving cell becomes smaller than the absolute thresholdvalue, iii) the event that a measurement value of a neighboring cellbecomes greater than the measurement value of the serving cell as muchas an offset value, iv) the event that the measurement value of theneighboring cell becomes greater than the absolute threshold value, andv) the event that the measurement value of the serving cell becomessmaller than the absolute threshold value and the measurement value ofthe neighboring cell becomes greater than another absolute thresholdvalue. In this case, the measurement value may be the aforementionedRSRP, etc. In a carrier aggregation environment, the serving cell maymean a PCell or SCell depending on the event.

The source base station may determine whether the user equipment willperform handover and determine the target base station to which the userequipment will perform handover (S603).

The source base station may transmit a handover request message to thetarget base station to perform handover (S605). For example, the sourcebase station provides radio resource control (RRC) context informationof the user equipment to the target base station.

The target base station determines whether to grant handover of the userequipment, based on the RRC context information (S607).

If handover of the user equipment is granted, the target base stationtransmits a handover (HO) request acknowledge message to the source basestation (S609).

The source base station that has received the HO request acknowledgemessage transmits an RRC connection reconfiguration message to the userequipment to command the user equipment to perform the HO procedure(S611). The RRC connection reconfiguration message may include radioresource configuration information, security configuration, cell ID(C-RNTI), which are commonly applied to the user equipments served bythe target base station. For example, the RRC connection reconfigurationmessage may include at least one of a Measurement Configuration IE(information element), a Mobility Control IE, a Radio ResourceConfiguration IE (e.g., RBs of the target base station, MACConfiguration, Physical Channel Configuration, and SIB that includessystem information of the target base station), and SecurityConfiguration IE. Table 1 illustrates a part extracted from the MobilityControl IE.

TABLE 1 MobilityControlInfo ::=  SEQUENCE {   targetPhysCellId  PhysCellId,   carrierFreq CarrierFreqEUTRA OPTIONAL,  -- CondHO-toEUTRA2 carrierBandwidth   CarrierBandwidthEUTRA   OPTIONAL,  --Cond HO-toEUTRA   additionalSpectrumEmission  AdditionalSpectrumEmission   OPTIONAL,  -- Cond HO-toEUTRA   t304  ENUMERATED { ms50, ms100, ms150, ms200, ms500, ms1000, ms2000,spare1},   newUE-Identity C-RNTI,   radioResourceConfigCommon  RadioResourceConfigCommon,   rach-ConfigDedicated  RACH-ConfigDedicated   OPTIONAL,  -- Need OP   ...,   [[carrierFreq-v9e0   CarrierFreqEUTRA-v9e0   OPTIONAL  -- Need ON   ]],  [[ drb-ContinueROHC-r11   ENUMERATED {true}   OPTIONAL  -- Cond HO  ]] }

Table 2 illustrates a part extracted from radioResourceConfigCommon IEwithin the Mobility Control IE.

TABLE 2 RadioResourceConfigCommonSIB ::= SEQUENCE {   rach-ConfigCommonRACH-ConfigCommon,   bcch-Config   BCCH-Config,   pcch-Config  PCCH-Config,   prach-Config PRACH-ConfigSIB,   pdsch-ConfigCommonPDSCH-ConfigCommon,   pusch-ConfigCommon PUSCH-ConfigCommon,  pucch-ConfigCommon PUCCH-ConfigCommon,   soundingRS-UL-ConfigCommonSoundingRS-UL-ConfigCommon,   uplinkPowerControlCommonUplinkPowerControlCommon,   ul-CyclicPrefixLength UL-CyclicPrefixLength,  ...,   [[ uplinkPowerControlCommon-v1020UplinkPowerControlCommon-v1020 OPTIONAL  -- Need OR   ]] }RadioResourceConfigCommon ::= SEQUENCE {   rach-ConfigCommonRACH-ConfigCommon OPTIONAL,  -- Need ON   prach-Config PRACH-Config,  pdsch-ConfigCommon PDSCH-ConfigCommon OPTIONAL,  -- Need ON  pusch-ConfigCommon PUSCH-ConfigCommon,   phich-Config PHICH-ConfigOPTIONAL,  -- Need ON   pucch-ConfigCommon PUCCH-ConfigCommonOPTIONAL,  -- Need ON   soundingRS-UL-ConfigCommonSoundingRS-UL-ConfigCommon OPTIONAL,  -- Need ON  uplinkPowerControlCommon UplinkPowerControlCommon OPTIONAL,  -- NeedON   antennaInfoCommon AntennaInfoCommon OPTIONAL,  -- Need ON   p-MaxP-Max OPTIONAL,  -- Need OP   tdd-Config TDD-Config OPTIONAL,  -- CondTDD   ul-CyclicPrefixLength UL-CyclicPrefixLength,   ...,   [[uplinkPowerControlCommon-v1020 UplinkPowerControlCommon-v1020OPTIONAL  -- Need ON   ]],   [[ tdd-Config-v1130 TDD-Config-v1130OPTIONAL  -- Cond TDD3   ]] }

Table 3 illustrates a RACH-ConfigDedicated IE within the MobilityControl IE.

TABLE 3 RACH-ConfigDedicated ::= SEQUENCE { ra-PreambleIndex INTEGER(0..63), ra-PRACH-MaskIndex INTEGER (0..15) }

The user equipment that has received the RRC connection reconfigurationmessage may be detached from the existing cell (that is, source basestation), and may perform a synchronization with a new cell (that is,target base station) (S613).

Since the source base station knows a target base station to which theuser equipment will perform handover, the source base station forwards abuffered packet, which will be transmitted to the user equipment, to thetarget base station (S615).

The source base station transmits a sequence number (SN) status transfermessage to the target base station to forward buffered data or packet tothe target base station (S617).

In the meantime, the user equipment that has received the RRC connectionreconfiguration message initiates a non-contention based random accessprocedure to the target base station. For example, the user equipmenttransmits a random access preamble to the target base station (S619).The user equipment randomly selects one random access preamble from aset of random access preambles indicated through a handover command, andtransmits the random access preamble by selecting a physical RACH(PRACH) resource that may transmit the random access preamble. Thetarget base station transmits uplink resource allocation information andtiming advance (TA) information for uplink synchronization to the userequipment through a medium access control (MAC) message or RRC messagein response to the random access preamble (S621).

The user equipment transmits RRC connection reconfiguration completemessage to the target base station based on the uplink resourceallocation information and the TA information (S623).

If the target base station receives the RRC connection reconfigurationcomplete message from the user equipment, the target base stationtransmits UE context release message, which requests removal ofinformation related to the user equipment, to the serving base station(S625).

The serving base station that has received the UE context releasemessage releases the resource for the user equipment and completes thehandover procedure (S627).

The aforementioned handover procedure of the user equipment iscategorized into a handover preparation procedure, a handover executionprocedure, and a handover completion procedure. The maximum time isrequired for the handover execution procedure. The handover executiontime means the time from the time when the user equipment receives theRRC connection reconfiguration message from the source base station(S611) to the time when the base station receives a handover connectionreconfiguration complete message from the user equipment (S623). Whenthe user equipment processes the RRC connection reconfiguration messageduring the handover execution procedure, the user equipment interruptsconnection with the source base station and interrupts data receptionfrom the source base station. Before the user equipment performssynchronization with the target base station and configures connection,the source base station forwards downlink data of the user equipment tothe target base station. Such data forwarding is performed more quicklythan reconfiguration of radio resources. Accordingly, the data forwardedto the target base station are stored in a buffer of the target basestation and are on standby to be transmitted to the user equipment untilthe UE is ready to receive data from the target base station.

As described above, the time required from the time when the userequipment that has received the RRC connection reconfiguration messageis detached from the source base station to the time when the userequipment completes RRC connection reconfiguration throughsynchronization with the target base station will be defined as datainterruption time. In the current mobile communication system, the datainterruption time is about 10.5 ms, and detailed time of each process isillustrated in Table 4 as follows.

TABLE 4 Description Time [ms] Radio Synchronization to the target cell 1Average delay due to RACH scheduling period 0.5 (1 ms periodicity) RACHPreamble 1 Preamble detection and transmission of RA response 5 (Timebetween the end RACH transmission and UE's reception of scheduling grantand timing adjustment) Decoding of scheduling grant and timing alignment2 Transmission of DL Data 1 Total delay 10.5

Moving Cell

In a hierarchical cell structure of a macro cell and a small cell, amoving cell may be considered as one type of the small cell. Forexample, the moving cell may be a small base station provided in adevice (for example, transportation means such as bus, train, or smartcar) which is physically moving, whereas the macro cell may be a fixedcell in the same manner as before.

Since the fixed cell (or macro cell) or the moving cell form thehierarchical cell structure, the moving cell and the user equipment maybe similar to each other in view of the macro cell. However, the movingcell should transmit and receive aggregated traffic of large capacitybased on a plurality of user equipments unlike a normal user equipment.Accordingly, a radio backhaul link that supports aggregated traffic oflarge capacity between the moving cell and the fixed cell is formed.

In the meantime, since the moving cell serves the user equipments, inview of the user equipments, the moving cell is regarded as the servingcell not another user equipment. The moving cell provides the userequipments with group mobility through physical movement and handover,wherein the user equipments are served by the moving cell. In-bandcommunication within the moving cell may support a full duplex mode.

As illustrated in Table 5, various types of moving cells may beconsidered. In this case, respective features according to the types ofthe moving cells should be considered.

TABLE 5 Access Link Category Backhaul Distance Mobility Moving PatternUser Load Public Long Wide speed range Fixed Medium/High TransportationSmart Car Medium/Short Wide speed range Arbitrary Low/Medium PersonalCell Various Low speed range Arbitrary Low/Medium

For example, a moving pattern of the moving cell is regularly moved inaccordance with a predetermined path in case of public transportation,whereas irregular movement may occur in case of a smart car or apersonal cell. In this way, a moving distance, moving speed or trafficsize may be varied depending on the type of the moving cell.

Handover of the user equipment is performed in such a manner that thesource base station receives measurement reports of neighboring basestations from the user equipment and the source base station commandsthe user equipment to perform handover through negotiation with thetarget base station. Accordingly, handover of the user equipment isnetwork initiated.

However, it is difficult to apply such a handover procedure of the userequipment to handover of the moving cell as it is. This is because thathigh reliability of communication service and less data interruptiontime are more required for the moving cell than handover of anindividual user equipment as the moving cell serves a plurality of userequipments. Since the user equipment that has accessed the moving cellrecognizes the moving cell as its serving cell, the moving cell shouldbe transparent and robust to a change of a radio channel environmentbased on its movement in providing services to the user equipments. Asillustrated in Table 4, delay of 10.5 ms occurs in handover of the userequipment, and it is preferable that the handover procedure of themoving cell is completed more quickly than 10.5 ms. The moving cellshould complete handover by quickly sensing the fixed cell, whereby aneffect due to handover, such as data interruption, may reach the userequipment served by the moving cell within the minimum range.

Handover of Moving Cell

According to one embodiment of the present invention, the moving celldetermines and triggers handover to minimize its handover delay. Forexample, the moving cell directly accesses the target base stationwithout measurement report to the source base station. The moving celldirectly determines the target base station based on the measurementresult and initiates handover, whereby the data interruption time may bereduced.

FIG. 8 is a diagram illustrating a handover procedure of a moving cellaccording to one embodiment of the present invention.

Referring to FIG. 8, the source base station transmits a measurementcontrol message to the moving cell (S800). The measurement controlmessage may be transmitted to the moving cell through RRC signaling. Themeasurement control message includes information required to performmeasurement of a serving cell and neighboring cells. For example,time/frequency/period of which measurement is performed by the movingcell, information (e.g., cell ID) on the serving cell or the neighboringcells, which are measurement targets, a black list listing cells ofwhich measurement is prohibited, frequency band (intra-frequency,inter-frequency) which is a measurement target, and information on ameasurement gap for inter-frequency measurement may be transmittedthrough the measurement control message.

In the meantime, the measurement control message may include informationon an event for triggering handover. If multiple carriers are configuredfor the moving cell, the event for triggering handover may be configuredfor each carrier. Meanwhile, the aforementioned events for measurementreport of the user equipment may be reused as events for triggeringhandover of the moving cell. Unlike this, an event for triggeringhandover of the moving cell may newly be defined.

Meanwhile, the moving cell should perform a random access procedure withthe target base station to access the target base station. If the movingcell performs the random access procedure based on contention such asinitial access instead of handover, a random access preamble and/orRA-RNTI of the moving cell may collide with a random access preambleand/or RA-RANTI of another user equipment or another moving cell duringthe random access procedure. For example, since RA-RNTI is an identifierdetermined as a time/frequency function for transmitting the randomaccess preamble, if the random access preamble transmitted from themoving cell is the same as that transmitted from another moving cell oruser equipment, RA-RNTI contention occurs and a contention resolutionprocedure is performed. Since the contention resolution procedure causestime delay in performing handover, it is preferable that anon-contention based random access is performed.

In order that the moving cell performs the non-contention based randomaccess, the measurement control message may include the random accesspreamble which will be transmitted from the moving cell. For example,the measurement control message may include a random access preambleallocated from the target base station to the moving cell. Meanwhile,since the target base station is determined by the moving cell, thesource base station cannot know a neighboring cell which will be thetarget base station, at the time when the source base station transmitsthe measurement control message. Accordingly, the source base stationmay include random access preambles of a plurality of neighboring cellsin the measurement control message. In this case, the random accesspreambles and identifiers of the neighboring cells may be mapped intoone another and then may be included in the measurement report message.

In the meantime, the random access preambles allocated from theneighboring cells may previously be stored in the source base station.For example, the random access preamble B previously allocated from theneighboring base station A may be stored in the source base station. Ifthe source base station forwards the random access preamble B to themoving cell C through the measurement control message, the source basestation may previously notify the neighboring base station A that themoving cell C transmits the random access preamble B. At this time,context information of the moving cell, which includes ID of the movingcell, may be forwarded to the neighboring cell.

According to another embodiment, the moving cell performs contentionbased random access, and the random access preamble may be divided intoa random access preamble type for the moving cell and a random accesspreamble type for the user equipment to prevent the moving cell fromperforming contention with the user equipment. At this time, althoughthe random access procedure of the moving cell may collide with therandom access procedure of another moving cell, if the number ofentities of the moving cell is small, the possibility of collision islowered. If the moving cell performs the contention based random access,the random access preamble is not included in the measurement controlmessage.

The moving cell that has received the measurement control messageperforms measurement, and determines whether to perform handover (S805).For example, the moving cell measures the serving cell and theneighboring cell based on the measurement control message, anddetermines whether to perform handover based on the measurement result.According to one embodiment, the moving cell may determine whether toperform handover and request the source base station of handover.However, according to another embodiment, if the moving cell determinesto perform handover, the moving cell may also determine the target basestation by itself. That is, the moving cell may determine the targetbase station among the neighboring cells based on the measurementresult. The measurement result of the neighboring cells may be RSRPand/or RSRQ, which is acquired through RS transmitted from theneighboring cell.

In the meantime, as the moving cell physically moves, the measurementvalues of RSRP and RSRQ may dynamically be varied. Accordingly, insteadof using RSRP/RSRQ measured at one time only to determine the targetbase station, the neighboring cell, which provides optimized RSRP/RSRQin accordance with the moving direction of the moving cell, may bedetermined as the target base station. For example, even thoughRSRP/RSRQ of the first neighboring cell is greater than RSRP/RSRQ of thesecond neighboring cell at the first time, if RSRP/RSRQ of the firstneighboring cell at the second time after the first time becomes smallerthan RSRP/RSRQ of the second neighboring cell, it may be regarded thatthe moving cell moves to be far away from the first neighboring cell andto be close to the second neighboring cell. In this way, it ispreferable that the moving cell determines the target cell byaccumulating the measurement results of the neighboring cells, which areacquired at different times.

For example, if an increased slope of RSRP/RSRQ of the secondneighboring cell exceeds a predetermined threshold value, the secondneighboring cell may be determined as the target base station, andhandover to the second base station may be triggered. At this time,although not limited to, the predetermined threshold value maypreviously be set in the moving cell, may be acquired from themeasurement values, or may be forwarded from the source base station tothe moving cell through the measurement control message. For example,the event for triggering handover may include, but not limited to, atleast one of i) the event that a decreased slope of the measurementvalue of the serving cell is greater than an absolute threshold value,ii) the event that the measurement value of the serving cell becomessmaller than the absolute threshold value, iii) the event that theincreased slope of the measurement value of the neighboring cell becomesgreater than the decreased slope of the measurement value of the servingcell as much as offset value, iv) the event that the increased slope ofthe measurement value of the neighboring cell becomes greater than theabsolute threshold value, and v) the event that the measurement value ofthe serving cell becomes smaller than the absolute threshold value andthe increased slope of the measurement value of the neighboring cellbecomes greater than another absolute threshold value. In this way, inselecting the target base station and determining whether to performhandover, variation per time of RSRP/RSRQ may be considered.

The moving cell acquires downlink synchronization for handover to thetarget base station, and transmits a handover request message, whichincludes a random access preamble (S810). At this time, the moving cellacquires downlink synchronization to the target base station withoutreleasing connection with the source base station and transmits therandom access preamble. Accordingly, for the user equipments that haveaccessed the moving cell, the moving cell may transmit and receiveuplink and downlink data to and from the source base station. Thehandover request message may include information on context of themoving cell. In another embodiment, the source base station may transmitthe information on context of the moving cell to the neighboring basestations, whereby the target base station may acquire the information oncontext of the moving cell.

The target base station performs grant control for handover of themoving cell based on the information on context of the moving cell(S815).

If handover of the moving cell is granted, the target base stationtransmits resource for UL transmission and TA information to the movingcell (S820). Also, the target base station transmits a handover acceptmessage, which indicates handover grant of the moving cell, to thesource base station (S825). The handover accept message may includeE-RAB information which is accepted or not accepted by the target basestation.

The source base station that has received the handover accept messagetransmits RRC connection reconfiguration message to the moving cell(S830). The moving cell performs a detachment procedure from the sourcebase station if the RRC connection reconfiguration message is received.Accordingly, data interruption is started in accordance with receptionof the RRC connection reconfiguration message.

In the meantime, C-RNTI or random access preamble, which is allocatedfrom the neighboring base stations in addition to the target basestation, may be stored in the moving cell in a state that C-RNTI orrandom access preamble is not used. The source base station may commandthe moving cell to delete the C-RNTI or random access preamble allocatedfrom the neighboring base stations through the RRC connectionreconfiguration message. The moving cell deletes the C-RNTI or randomaccess preamble allocated from the neighboring base stations inaccordance with the command of the RRC connection reconfigurationmessage.

Meanwhile, according to one embodiment, in addition to the target basestation, the neighboring base stations may previously acquire and storecontext information of the moving cell or information on a handoverhistory of the moving cell from the source base station. The source basestation may command the neighboring base stations in addition to thetarget base station to delete the context information of the moving cellor the information on the handover history of the moving cell inaccordance with reception of the handover accept message.

The source base station transmits a status transfer message to thetarget base station in accordance with reception of the handover acceptmessage (S840). The moving cell completes RRC configuration with thetarget base station based on the RRC connection reconfiguration messageand transmits a connection reconfiguration complete message to thetarget base station (S845).

Meanwhile, the handover accept message may be forwarded to the sourcebase station immediately after grant control is performed by the targetbase station.

In the aforementioned handover method of the moving cell, since themoving cell tries connection with the target base station in a statethat connection with the source base station is maintained, datainterruption time is reduced. For example, the moving cell triesconnection with the target base station while receiving data from thesource base station. Since the moving cell is detached from the sourcebase station after trying connection with the target base station, thetime required for the moving cell to access the target base station byinterrupting connection with the source base station is reduced. If thehandover method of the user equipment is a “Break before Make” typeprocedure, the handover method of the moving cell may be regarded as a“Make before Break” procedure. Table 6 illustrates the time required forhandover of the moving cell in accordance with one embodiment of thepresent invention.

TABLE 6 Description Time [ms]

Decoding of scheduling grant and timing alignment 2 Transmission of DLData 1 Total delay 3

FIG. 9 is a diagram illustrating a handover procedure of a moving cellaccording to another embodiment of the present invention. Repeateddescription of FIG. 8 will be omitted, and a difference from theembodiment of FIG. 8 will be described mainly.

Referring to FIG. 9, the source base station transmits the measurementcontrol message to the moving cell (S900). The moving cell that hasreceived the measurement control message performs measurement, anddetermines whether to perform handover (S905). The moving cell performsdownlink synchronization for handover to the target base station, andtransmits a handover request message, which includes a random accesspreamble (S910). The target base station performs grant control forhandover of the moving cell (S915). If handover of the moving cell isgranted, the target base station transmits resource for UL transmissionand TA information for uplink synchronization to the moving cell (S920).Similarly to FIG. 8, the moving cell performs downlink synchronizationwith the target base station in a state that connection with the sourcebase station is not interrupted, and transmits the random accesspreamble.

The moving cell transmits RRC connection reconfiguration message to thetarget base station (S925). The RRC connection reconfiguration messagemay include Temporary Mobile Subscriber Identity (TMSI) of the movingcell, C-RNTI information of the moving cell, which is previouslyacquired from the source base station or the target base station, and aCause value of RRC connection reconfiguration request. For handover ofthe moving cell, the Cause value of RRC connection reconfigurationrequest is set to a value corresponding to handover.

The target base station transmits RRC connection reconfiguration messageto the moving cell in response to the RRC connection reconfigurationrequest (S930). That is, the target base station directly transmitsinformation required for RRC reconfiguration with the target basestation to the moving cell. In the embodiment of FIG. 8, the informationrequired for RRC reconfiguration with the target base station isforwarded to the moving cell through the source base station, whereasthe target base station directly transmits the RRC reconfigurationmessage in the embodiment of FIG. 9. The RRC connection reconfigurationmessage may include an identifier (e.g., C-RNTI) allocated from theneighboring base stations in addition to the target base station andstored in the moving cell and indication indicating deletion of therandom access preamble. The moving cell deletes the identifier allocatedfrom the neighboring base stations except the target base station andthe random access preamble.

The moving cell transmits RRC reconfiguration complete message to thetarget cell if RRC reconfiguration is completed (S935).

The target base station transmits a handover accept message to thesource base station to notify the source base station that handover ofthe moving cell has been accepted and completed (S904). The handoveraccept message may include E-RAB information which is accepted or notaccepted by the target base station. The source base station that hasreceived the handover accept message is detached from the moving cell.In order that the source base station is detached from the moving cell,RRC signaling still maintained with the moving cell may be used. Forexample, after receiving the handover accept message from the targetbase station, the source base station may transmit RRC signaling to themoving cell, whereby the moving cell and the source base station may bedetached from each other. Alternatively, the source base station isdetached from the moving just after receiving RRC connection setupmessage from the moving cell.

In this embodiment, since the moving cell configures RRC connection withthe target base station and is detached from the source base station,the data interruption time does not occur. Meanwhile, the embodiment ofFIG. 9 is favorably applied to a case where a moving path of the movingcell has a certain pattern or may be predicted.

Hereinafter, embodiments of a handover method considering a moving pathof a moving cell will be described.

Handover Pre-Processing of Moving Cell Having Fixed Moving Path

If the moving cell moves along a bus line, a train track, etc., its nextlocation may be predicted. In this way, the case where movement of themoving cell has a pattern, is previously notified, or is regular orconsistent will be referred to as a fixed moving path.

The base stations located on the fixed moving path may share informationon the moving path of the moving cell. A list of base stations to whichthe moving cell performs handover and information on the order may beshared by each base station.

In the handover method of the user equipment as shown in FIG. 7, whenperforming measurement, the user equipment does not acquire systeminformation (e.g., MIB/SIB) of the target base station or RRCconfiguration. The user equipment acquires system information of thetarget base station and information on RRC configuration through thehandover command message, that is, RRC connection reconfigurationmessage. Meanwhile, the system information of the target base station,which is acquired through the RRC connection reconfiguration message, isRadioResourceConfigCommonSIB IE included in SIB 2. Accordingly, in orderthat the user equipment acquires other system information of the targetbase station, the user equipment should receive SIBs (e.g., SIB 1 to SIB14) transmitted through a downlink shared channel after synchronizingwith the target base station.

According to one embodiment of the present invention, the moving cellmay acquire the system information of the target base station orinformation on RRC configuration in advance before initiating handover.For example, the moving cell may acquire the system information of thetarget base station or information on RRC configuration beforeperforming measurement of the neighboring cell. Accordingly, the movingcell does not need to acquire the system information of the target basestation separately after performing handover to the target base station.As a result, the data interruption time of the user equipments that haveaccessed the moving cell may be minimized.

As described above, in order that the user equipment performs thenon-contention based random access, a dedicated random access preambleof the user equipment, which is used in the target base station, andC-RNTI should be allocated to the moving cell. Meanwhile, the sourcebase station may previously forward context information on the movingcell, information on a handover history, etc. to the target base stationor neighboring base stations which are candidates of the target basestation before handover of the moving cell is performed. In this way, ahandover pre-processing procedure, which includes previous exchange ofinformation, may be performed.

If the moving path of the moving cell is fixed, at least two or more ofbase stations located on the moving path may share an identifier (e.g.,C-RNTI) of one moving cell. For example, the identifier of the movingcell, which is used prior to handover, may be reused even at the targetbase station after handover is performed. Referring to FIG. 10, the basestations located on the moving path allocate one permanent identifierfor the moving cell. Generally, since C-RNTI is unique ID within theserving cell only, if the user equipment performs handover, the userequipment should be allocated with new C-RNTI which will be used withinthe target base station. ID of the moving cell according to theembodiment of the present invention is unique ID within the basestations on the moving path.

Referring to FIGS. 10 and 11, it is assumed that the moving cell iscurrently located at the first base station and is scheduled to performhandover to the second base station in accordance with the fixed movingpath. That is, it is assumed that the first base station is the servingbase station and the second base station is the target base station.

The first base station may transmit a handover pre-processing requestmessage, which includes at least one of ID of the moving cell, contextinformation of the moving cell and a handover history of the movingcell, to the second base station (S1100). For example, if the movingcell configures RRC connection to the first base station throughhandover or initial access, the first base station may transmit thehandover pre-processing request message to the second base stationbefore the handover procedure of the moving cell is initiated. Theinformation on the handover history may be used to identify a basestation to which the moving cell will perform handover.

The second base station transmits a handover pre-processing responsemessage, which includes the dedicated random access preamble of themoving cell, the system information of the target base station and RRCconfiguration information of the target base station, to the first basestation in response to the handover pre-processing request message(S1105). The RRC configuration information of the target base stationmay include at least one of MobilityControlInfo IE, MeasConfig IE, andSecurityConfigHO IE.

The first base station transmits at least one of the random accesspreamble included in the handover pre-processing response messagereceived from the second base station, the system information of thetarget base station and the RRC configuration information of the targetbase station to the moving cell (S1110).

In the meantime, permanent ID allocation of the moving cell may beperformed uniquely for all the base stations existing in the movingpath, and may be performed uniquely in a unit of tracking area (TA).

Meanwhile, if a plurality of cells exist in accordance with the movingpath, measurement overhead of the moving cell may be increased. In orderto reduce such measurement overhead, the source base station or thetarget base station may transmit a Measurement Configuration(Measurement Control) message, which includes measurement object (whitecell) to be measured by the moving cell or information (black cell) notto be measured by the moving cell, to the moving cell through RRCsignaling. At this time, the white cell may be, but not limited to,located on the moving path while the black cell may be, but not limitedto, located at a long distance from the moving path.

Handover Pre-Processing of Moving Cell Having Non-Fixed Moving Path

If the moving path is not consistent, an exemplary embodiment forperforming a handover pre-processing procedure will be described. Therepeated description of the aforementioned description will be omitted.

Candidates of potential target base stations, to which the moving cellmay perform handover, will be referred to as candidate base stations orcandidate cells.

If the moving path is not fixed, it is difficult to allocate a uniquepermanent identifier from the base station on the moving path. That is,this is because that it is not possible to know a base station whichwill be located on the moving path. If all the base stations allocateunique ID to the moving cell, for the moving cell, the base stations, towhich the moving cell does not perform handover, should allocate ID.However, since the number of IDs is restrictive, the method forallocating unique ID from all the base stations to the moving cell isnot efficient in view of usage of the ID.

Accordingly, in the embodiment that the moving path is not fixed, it isassumed that unique ID (e.g., C-RNTI) is allocated within each servingbase station (or cell) only. In the handover pre-processing procedure,the serving base station may request the candidate base stations of IDwhich will be used by the moving cell after handover is performed,thereby previously notifying the moving cell of the ID before handoverof the moving is initiated.

A problem as to how the source base station determines the candidatebase stations may occur. According to one embodiment of the presentinvention, E-SMLC (Evolved Serving Mobile Location Center) in the mobilecommunication system may be used. The E-SMLC is a network node thatmanages mobility and location measurement of the user equipment or themoving cell. The E-SMLC may identify what neighboring base stationsexist based on the location of the moving cell. The E-SMLC determinescandidate base stations, to which the moving cell may potentiallyperform handover, based on the information received from the source basestation, for example, information on a handover history of the movingcell or ID of the moving cell. The source base station identifies thepotential handover target base stations of the moving cell through theinformation received from the E-SMLC, and transmits the handoverpre-processing request message to the target base stations.

In another embodiment, the moving cell may determine the candidate basestations. For example, if the moving cell perceives the potentialhandover base stations, that is, the candidate base stations throughmeasurement, the moving cell transmits information on the candidate basestations to the source base station. The source base station transmitsthe handover pre-processing request message to the candidate basestations.

Referring to FIGS. 12 and 13, it is assumed that the moving cell iscurrently located at eNB 4 and eNBs 1, 5 and 7 are the candidate basestations. The moving cell accesses the source base station throughinitial access or handover.

If the moving cell accesses the source base station, the source basestation transmits a neighboring base station information request(Neighbor eNB/Cell Info Request) message requesting information on thecandidate base stations to the E-SMLC (S1300). The neighboring basestation information request message may include information on ahandover history of the moving cell. The message transmitted andreceived between the source base station and the E-SMLC may be a formatbased on LTE positioning Protocol A (LPPa). The LPPa is defined by the3GPP TS 36.455.

The E-SMLC determines candidate base stations determined that the movingcell may potentially perform handover, based on the information on thehandover history of the moving cell. The E-SMLC may restrict the numberof candidate base stations to increase a hit ratio of the target basestation when determining the target base station. The E-SMLC transmits aneighboring base station information response message, which includesinformation on the determined candidate base stations, to the sourcebase station (S1305).

The source base station transmits the handover pre-processing requestmessage, which includes context information of the moving cell andinformation on the handover history of the moving cell, to the candidatebase stations including the target base station (S1310).

The candidate base stations transmit the handover pre-processingresponse message, which includes at least one of C-RNTI for the movingcell, the random access preamble, system information of the candidatebase stations, and RRC configuration information of the candidate basestations, to the source base station (S1315).

The source base station transmits the information received from thecandidate base stations to the moving cell (S1320). Even in this case,the source base station or the candidate base stations may transmitinformation on cells which are measurement targets and information oncells which should not be measured, to the moving cell.

FIG. 14 is a diagram illustrating a handover procedure of a moving cellaccording to other embodiment of the present invention. Referring toFIG. 14, a moving cell, a mobile terminal, a source base station and atarget base station are shown. It is assumed that the moving cell andthe mobile terminal are served respectively by the source base station.The repeated description of the aforementioned embodiments will beomitted. Accordingly, this embodiment will be described with referenceto the aforementioned descriptions.

A moving entity accesses the source base station. The moving entity is anetwork entity having mobility and includes the moving cell and themobile terminal. The moving entity may initially access the source basestation or access the source base station by performing handover fromanother base station.

If the moving entity accesses the source base station, the source basestation determines whether the moving entity is the mobile terminal orthe moving cell (S1400).

If the moving entity is the moving cell, the source base stationtransmits the handover pre-processing request message to the neighboringbase stations including the target base station (S1430). The handoverpre-processing request message may include information on a handoverhistory of the moving cell.

The neighboring base stations transmit the handover pre-processingresponse message to the source base station (S1435). The handoverpre-processing response message may include at least one of ID whichwill be used after the moving cell performs handover and information onrandom access preambles allocated from the neighboring base stations.

The source base station transmits a measurement configuration message,which includes measurement configuration information on the neighboringbase stations and information required to determine whether the movingcell initiates handover, to the moving cell (S1440). The source basestation may transmit the information acquired through the handoverpre-processing response message to the moving cell.

The moving cell determines whether to initiate handover and the targetbase station based on the result of measurement of the neighboring basestations. If initiation of handover and the target base station aredetermined, the moving cell performs the non-contention based randomaccess procedure with the target base station (S1445). That is, themoving cell transmits the allocated random access preamble to the targetbase station before handover from the target base station is initiated.

If the target base station accepts handover, the target base stationtransmits a handover accept message to the source base station (S1450).

The source base station transmits RRC connection reconfigurationmessage, which indicates reconfiguration of RRC connection, to themoving entity (S1455).

In the meantime, the source base station returns the IDs for the movingcell, which are allocated from the other neighboring base stationsexcept the target base station, and/or the random access preamble to theother neighboring base stations in accordance with reception of thehandover accept message. Also, the moving cell deletes the random accesspreamble allocated from the other neighboring base stations except thetarget base station.

The aforementioned handover of the moving cell may be performedtransparently to the mobile terminals served by the moving cell.

If the moving entity is the mobile terminal, the source base stationtransmits a measurement configuration message, which includesmeasurement configuration on the neighboring base stations, to themoving entity (S1405). That is, the source base station does nottransmit information for determining handover initiation to the mobileterminal. This is because that the serving base station determineshandover initiation and the target base station if the moving entity isthe mobile terminal. On the other hand, if the moving entity is themoving cell, the moving cell determines handover initiation and thetarget base station.

The source base station receives a measurement report from the mobileterminal (S1410). The source base station determines handover initiationof the mobile terminal and the target base station based on themeasurement report. In this way, handover initiation and the selectionscheme of the target base station are determined according to whetherthe moving entity is the mobile terminal or the moving cell.

The source base station transmits a handover request message to thetarget base station (S1415), and receives a handover accept message fromthe target base station (S1420).

The source base station transmits RRC connection reconfigurationmessage, which indicates reconfiguration of RRC connection, to themobile terminal (S1425). The RRC connection reconfiguration messagetransmitted to the mobile terminal may include the random accesspreamble allocated from the target base station. That is, if the movingentity is the mobile terminal, the random access preamble is provided tothe mobile terminal after handover is initiated through the RRCconnection reconfiguration message. On the other hand, if the movingentity is the moving cell, the random access preamble allocated from thetarget base station is provided to the moving cell before handover isinitiated.

In the meantime, handover of the mobile terminals in the moving cell maybe performed in accordance with the aforementioned handover procedure ofthe mobile terminal. For example, if the mobile terminal served by themoving cell performs handover, the moving cell releases RRC connectionof the mobile terminal before the mobile terminal transmits the randomaccess preamble.

FIG. 15 is a diagram illustrating a handover pre-processing procedure ofa moving cell according to other embodiment of the present invention.The repeated description of the aforementioned description will beomitted.

First of all, the source base station determines whether the moving cellhas a fixed moving path or a non-fixed moving path, based on theinformation on a handover history of the moving cell (S1500).

If the source base station determines that the moving cell has anon-fixed moving path, the source base station requests the E-SMLC ofneighboring base station information (S1505). For example, the sourcebase station transmits a message, which includes the information on ahandover history of the moving cell, to the E-SMLC.

The source base station transmits a neighboring base station informationresponse message, which includes a list of neighboring base stations(S1510). It is assumed that the list of neighboring base stationsincludes a target base station and a first neighboring base station.

The source base station transmits a handover pre-processing requestmessage to the target base station and the first neighboring basestation (S1515). The handover pre-processing request message may includeinformation on a handover history of the moving cell and contextinformation of the moving cell.

The source base station receives a handover pre-processing responsemessage from each of the target base station and the first neighboringbase station (S1520). The handover pre-processing response message mayinclude a random access preamble, ID of the moving cell and informationon RRC configuration.

The source base station forwards the handover pre-processing responsemessage to the moving cell before handover is initiated (S1525).

The moving cell transmits the random access preamble to the target basestation (S1530). Handover is initiated in accordance with transmissionof the random access preamble.

The target base station transmits a handover response message, whichindicates acceptance of handover, to the source base station (S1535).

The source base station returns the random access preamble, which is notused by the moving cell, to the first neighboring base station (S1540).Meanwhile, the source base station may return the ID allocated from thefirst neighboring base station to the moving cell together with therandom access preamble. The first neighboring base station gets therandom access preamble and the ID back from the moving cell, and mayallocate the random access preamble and the ID to another moving cell orthe mobile terminal. Also, the source base station may command the firstneighboring base station to delete the information on a handover historyof the moving cell and the context information of the moving cell.

The source base station may command the moving cell to delete the randomaccess preamble of the first neighboring base station, the RRCconfiguration of the first neighboring base station, the systeminformation of the first neighboring base station, and the ID allocatedfrom the first neighboring base station (S1545).

Meanwhile, if the moving cell has a fixed moving path, the source basestation may notify a base station to which the moving cell will performhandover. That is, the source base station may identify the target basestation.

The source base station transmits the handover pre-processing requestmessage to the target base station (S1550), receives the handoverpre-processing response message from the target base station (S1560),and forwards the handover pre-processing response message to the movingcell (S1560). If handover of the moving cell is initiated (S1565), thesource base station receives a handover response from the target basestation (S1570). Meanwhile, the ID allocated from the source basestation to the moving cell may be reused by the base stations located onthe fixed moving path.

FIG. 16 is a diagram illustrating a cell according to one embodiment ofthe present invention. The cell 1600 shown in FIG. 16 may be a movingcell or a fixed cell. If the cell 1600 is the fixed cell, the cell maybe a serving base station that serves the moving cell or a neighboringbase station located near the serving base station. The cell 1600 mayperform the operation of the moving cell or the fixed cell in theaforementioned embodiments.

The cell 1600 includes a plurality of antennas for MU-MIMO, a radiointerface 1601, a backhaul interface 1602, a memory 1603, and aprocessor 1604. The backhaul interface 1602 transmits and receives datato and from another cell or the base station through a backhaul link. Ifthe backhaul interface 1602 forms a radio backhaul link, the backhaulinterface 1602 and the radio interface 1601 may be implemented as onemodule. The radio interface 1601 transmits and receives signals to andfrom the mobile terminals through the plurality of antennas. Theprocessor 1604 controls the backhaul interface 1602, the radio interface1601, and the memory 1603.

First of all, it is assumed that the cell 1600 is a serving base stationof the moving cell and the serving base station performs pre-processingfor handover of the moving cell. At this time, the backhaul interfacetransmits a handover pre-processing request message, which includesinformation on a handover history of the moving cell to at least oneneighboring base station located near the serving base station. Thebackhaul interface receives a pre-processing response message, whichincludes at least one of a random access preamble allocated from theneighboring base station, RRC configuration of the neighboring basestation and system information of the neighboring base station, from theneighboring base station. The radio interface transmits thepre-processing response message to the moving cell before handover ofthe moving cell is initiated.

On the other hand, it is assumed that the cell 1600 is a neighboringbase station that performs pre-processing for handover of the movingcell. At this time, the backhaul interface receives the handoverpre-processing request message, which includes information on a handoverhistory of the moving cell, from the serving base station. The backhaulinterface transmits the pre-processing response message, which includesat least one of the random access preamble allocated from theneighboring base station, RRC configuration of the neighboring basestation and system information of the neighboring base station, to theserving base station. The pre-processing response message transmittedfrom the neighboring base station is forwarded to the moving cell beforehandover of the moving cell is initiated.

In accordance with another embodiment, when the cell 1600 is the servingbase station that supports handover of the moving entity, the radiointerface transmits a measurement configuration message, which includesmeasurement configuration regarding neighboring base stations, to themoving entity. The radio interface transmits RRC connectionreconfiguration message, which indicates reconfiguration of RRCconnection, to the moving entity. The backhaul interface receives ahandover accept message from the target base station, to which themoving entity will perform handover, among the neighboring basestations. At this time, handover initiation and a selection scheme ofthe target base station may be determined depending on whether themoving entity is a mobile terminal or a moving cell.

FIG. 17 is a diagram illustrating a structure of a user equipment and abase station according to one embodiment of the present invention.

The base station 10 may include a transmitter 11, a receiver 12, aprocessor 13, a memory 14, and a plurality of antennas 15. The pluralityof antennas 15 mean the base station that supports MIMO transmission andreception. The receiver 10 may receive various signals, data andinformation on an uplink from the user equipment. The transmitter 11 maytransmit various signals, data and information on a downlink to the userequipment. The processor 13 may control the overall operation of thebase station 10.

The processor 13 of the base station 10 may perform a function ofoperation-processing information received by the base station 10 andinformation to be transmitted to the outside. The memory 14 may storethe operation-processed information for a predetermined time, and may bereplaced with a buffer (not shown).

The user equipment 20 may include a transmitter 21, a receiver 22, aprocessor 23, a memory 24, and a plurality of antennas 25. The pluralityof antennas 25 mean the user equipment that supports MIMO transmissionand reception. The receiver 21 may receive various signals, data andinformation on the downlink from the base station. The transmitter 22may transmit various signals, data and information on the uplink to thebase station. The processor 23 may control the overall operation of theuser equipment 20.

The processor 23 of the user equipment 20 may perform a function ofoperation-processing information received by the user equipment 20 andinformation to be transmitted to the outside. The memory 24 may storethe operation-processed information for a predetermined time, and may bereplaced with a buffer (not shown).

The aforementioned embodiments according to the present invention may beimplemented by various means, for example, hardware, firmware, software,or their combination.

If the embodiments according to the present invention are implemented byhardware, the embodiments of the present invention may be implemented byone or more application specific integrated circuits (ASICs), digitalsignal processors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

If the embodiments according to the present invention are implemented byfirmware or software, the embodiments of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

It will be apparent to those skilled in the art that the presentinvention may be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is also obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by a subsequent amendment after theapplication is filed.

INDUSTRIAL APPLICABILITY

The aforementioned embodiments according to the present invention may beapplied to various wireless communication systems.

The invention claimed is:
 1. A method for supporting handover of amoving entity by a serving base station, the method comprising:receiving, from neighboring base stations, identifiers and random accesspreambles to be allocated to a moving cell; transmitting a measurementconfiguration message including measurement configuration regarding theneighboring base stations, to the moving entity; receiving a handoveraccept message from a target base station, to which the moving entitywill perform handover, among the neighboring base stations; andtransmitting a radio resource control (RRC) reconfiguration messageindicating reconfiguration of RRC connection, to the moving entity,wherein a handover initiation scheme and a target base station selectionscheme are determined according to whether the moving entity is a mobileterminal or the moving cell, and wherein when the moving entity is themoving cell, the serving base station returns the identifiers and randomaccess preambles to the neighboring base stations except for the targetbase station in accordance with the reception of the handover acceptmessage.
 2. The method according to claim 1, wherein: the serving basestation determines initiation of the handover and the target basestation if the moving entity is the mobile terminal, and the moving celldetermines initiation of the handover and the target base station if themoving entity is the moving cell.
 3. The method according to claim 1,wherein the measurement configuration message further includesinformation required for the moving cell to determine whether toinitiate the handover if the moving entity is the moving cell.
 4. Themethod according to claim 1, wherein: if the moving entity is the movingcell, a random access preamble allocated from the target base station isprovided to the moving cell before the handover is initiated, and if themoving entity is the mobile terminal, the random access preamble isprovided to the mobile terminal through the RRC connectionreconfiguration message after the handover is initiated.
 5. The methodaccording to claim 1, further comprising: determining whether the movingentity is the mobile terminal or the moving cell.
 6. The methodaccording to claim 1, wherein the handover of the moving cell isperformed transparently to mobile terminals served by the moving cell.7. A serving base station for supporting handover of a moving entity,the serving base station comprising: a radio interface configured totransmit a measurement configuration message including measurementconfiguration regarding neighboring base stations, to the moving entityand transmit a radio resource control (RRC) reconfiguration messageindicating reconfiguration of RRC connection, to the moving entity; abackhaul interface configured to receive, from the neighboring basestations, identifiers and random access preambles to be allocated to amoving cell and receive a handover accept message from a target basestation, to which the moving entity will perform handover, among theneighboring base stations; and a processor configured to control theradio interface and the backhaul interface, wherein a handoverinitiation scheme and a target base station selection scheme aredetermined according to whether the moving entity is a mobile terminalor the moving cell, and wherein when the moving entity is the movingcell, the processor returns the identifiers and random access preamblesto the neighboring base stations except for the target base station inaccordance with the reception of the handover accept message.
 8. Theserving base station according to claim 7, wherein: the serving basestation determines initiation of the handover and the target basestation if the moving entity is the mobile terminal, and the moving celldetermines initiation of the handover and the target base station if themoving entity is the moving cell.